The invention relates to screens for use in process flow stream applications where a significant pressure drop through the screen is often essential to provide adequate distribution. This use is quite different from well screen applications where it is usually desirable to maximize the amount of open area and provide a minimum pressure drop. An example of a conventional well screen can be seen in Johnson U.S. Pat. No. 2,046,458 wherein a profiled wire is helically wrapped around a plurality of longitudinal rods and welded to each one so as to define slots of a predetermined width between the successive wraps of wire. Where it is desired to use such a screen in an application requiring a substantial pressure drop through the screen, some of the slots may be welded closed or a perforated pipe or a plate that has been punched and rolled can be mounted inside the screen. Geske U.S. Pat. No. 4,096,911 discloses a cylindrical well screen which incorporates a plurality of apertured channel members having short leg portions which are welded to each wrap of wire, the area of the apertures preferably being at least as great as the open area of the slots. The channels are positioned longitudinally of the screen cylinder and parallel to its axis so as to resist large torsional loads in deep drilling which might destroy a screen of the conventional rod and wire type.
A typical example of a process application where a screen is used to retain catalyst particles is the moving bed reactor shown in Greenwood et al U.S. Pat. No. 3,706,536. In FIG. 9 of the patent a screen is shown as being backed up with a perforated baffle plate. The screen slots are sized to retain the catalyst and the baffle plate has perforations which distribute the reactant stream uniformly through the system and control its flow rate. The open areas of the screen and plate could rarely be the same since it is by having a smaller open area in the baffle plate that a sufficient back pressure is built up in the reactant stream to cause the stream to contact all of the catalyst uniformly. A similar need to retain particles and control distribution would also occur in a fluidized bed. Although it is theoretically possible to make the screen slots small enough to provide a significant pressure drop, it would be very difficult and expensive to achieve very small openings and such small openings might be very susceptible to clogging.